Need Help with Glow Plug retrofit on '76 300D "W115" - PeachParts Mercedes-Benz Forum

pdxwaker · #12 03-07-2007, 12:00 PM

Quote:

Originally Posted by samdon

I remember with the old series setup, the glow time was never constant. If it was relatively warm outside it would glow for a much shorter period of time - On a cold morning it might go for a good long period. What is the difference in voltage drop between the last plug in the old series and any one of the new parallel plugs? Should the wire possibly be connected as it was? I can't see this being the case as I know for a fact that it is not looking for battery voltage but how about the slightly lower, but climbing voltage seen at the plugs?

There should be an internal timer in the relay that prevents the plugs from cycling more than ~110 sec. I'm assuming this is used in conjunction with the temp sensor in the block (though I'm not certain because the online service manuals are down). If the temp sensor was truly connected to the last plug in series (toward the front of the car), it must have had some serious current flowing through it (~50A).

My guess is that the temp sensor is used only for the GP indicator light. For W123s, the glow plug light will go out but the plugs are still cycling. The GP light can be used as a 'ready to start' indicator. I know this isn't a W123 but your GP relay system was the predecesor to the W123...

Do you know if your replacement plugs are Bosch Duratherm? If so there shouldn't be a problem with forcing the 110 second max cycle time. Note that this cycle time can be shortened by just cranking the engine earlier (this is why there is a 'cranking' input on the relay... glow plug power is cut off when cranking). I would suggest grounding the temp sensor input to the relay and just forcing the max glow plug cycle time. Use your DMM and a stop watch to verify that the max cycle time is ~110 sec. It would also be good the verify that plug power is cut off when cranking starts.

Don't let this change worry you... all the older style W115 systems (non-relay) were completely manual... glow plug cycling time controlled by how long you pull the start knob. Essentially you'll be taking the 'auto temp' shutoff out of the control loop and creating more of a manual style control.